During operation of a typical conventional manual transmission, a reverse gear is engaged with the use of a sliding gear, coupler or another device when reverse drive is selected by the vehicle operator for interconnecting the reverse gear with the input shaft. Often the transmission is disengaged from one of the forward gears immediately prior to the selection of reverse gear. At such time, the input shaft is free to inertially rotate under a substantially no-load condition until, through friction and windage losses, the rotating inertia of the input shaft is dissipated and rotation in a forward direction is arrested.
If the gear selector lever is moved so as to attempt to interconnect the stationary reverse gear with the input shaft while the input shaft remains rotating in a direction corresponding with a forward drive gear, the teeth of the coupler or synchronizer sleeve will strike the non-moving clutch teeth associated with the reverse gear. The force applied manually by the vehicle operator to the coupler or synchronizer sleeve toward the direction of the engagement with the reverse gear results in an undesirable clattering as the tips of the sleeve teeth and those of the dog or clutch teeth are forced together, but unable to mesh because of their relative rotational speeds. This condition translates to an unpleasant grinding noise and feel to the vehicle operator. In certain instances, if the forces applied by the vehicle operator are sufficiently large, damage to the clutch teeth or those of the sleeve may result. Therefore, to avoid this condition, the vehicle operator is forced to wait several seconds for the input shaft to discontinue rotation before the gearshift lever can be moved quietly and smoothly to engage the reverse gear ratio.
Rather than resolution, modern technology has resulted in accentuation of this problem. Factors such as continued desires to improve vehicle gas mileage and to improve shifting operations in cold climates are the major contributors. In this regard, the introduction of low-viscosity lubricating oil into a manual transmission as a way to improve performance has further decreased the negative forces which naturally act on an input shaft to arrest rotational motion. As a result, the input shafts of modern transmissions are permitted to inertially rotate under substantially no-load conditions for increasingly longer durations.
One known technique to provide substantially equivalent rotational speeds between rotational parts is a fully synchronized gear arrangement. With respect to forward drive gears, this approach has been adopted as standard throughout the automotive industry. However, the cost and complexity associated with such an arrangement adapted to synchronize the reverse drive engagement has not been commercially justified, particularly in view of a need that is considered to be limited. In this regard, it can be generally assumed that reverse drive is selected when the vehicle is operating at low speed or when the drive wheels are fully stopped.
Nonetheless, any delay incurred after the neutral clutch pedal is depressed and before the shift lever can be moved to the reverse drive position to avoid causing gear clashing is objectionable. This is particularly true in small vehicles where high speed engines and manual transmissions are often used. Even at the idle speed of small modern engines, the forward gear ratios are driven at relatively high rotational speeds, have inherently high inertia, and require longer periods of delay before the reverse drive gear may be engaged. It follows that the chattering noise and grinding feedback discussed above could be virtually eliminated by ensuring that rotation of the input shaft was substantially arrested before attempting to engage the reverse drive gear.
Several prior devices have been developed in an attempt to eliminate the undesirable grinding noise and feel often encountered when shifting a manual transmission into reverse drive. For example, U.S. Pat. No. 4,294,338 to Simmons discloses a shift apparatus including a modified synchronizer which functions as a countershaft brake. A brake cone is non-rotationally fixed to the housing and pilots on a countershaft. Similarly, U.S. Pat. No. 4,598,599 to Ikemoto also discloses a shift apparatus including a modified synchronizer which functions as a countershaft brake. U.S. Pat. No. 4,225,024 to Kuzma discloses a locking device for reverse gear of a manual transmission. A friction ring is nonrotatably disposed on a drive shaft of the transmission and is slidable between a gear wheel and a dog disposed coaxially with respect to the driven shaft.
While these prior known devices have proven to be commercially acceptable, each is attendant with its drawbacks and inherent limitations. For example, some prior known devices creates the possibility that the cone many not position as readily with the synchronizer creating a blocking effect resulting in difficulty completing the engagement of the reverse gear ratio. Other prior known devices are associated with increased manufacturing, maintenance and assembly costs.
Thus, there remains a need in the art for a simple, inexpensive reverse brake for a manual transmission which is effective to eliminate gear chatter when a shifting into reverse from a forward gear.